1. Field of the Invention
This invention relates to a process of producing chlorine dioxide and, if desired, chlorine from hydrogen chloride and electrolytically produced alkali chlorate wherein the resulting residual solution is recycled to the electrolytic process by which new chlorate is produced.
2. Discussion of the Prior Art
It is known to use chlorine, chlorine dioxide and sodium hydroxide solution for bleaching pulp in pulp mills. Whereas chlorine and sodium hydroxide solution are commercially available, chlorine dioxide must be produced where it is used because it is explosive when concentrated. For this reason, it is known to produce chlorine dioxide by reduction of commercially available chlorate in a solution of a mineral acid which, in pulp mills, usually consists of sulfuric acid. In sulfate pulp mills, the resulting sodium sulfate has been used to make up for sodium sulfate which has been lost. Increased efforts to decrease the consumption of sulfates have been made for ecological reasons. As a result, it is no longer possible to use, in the pulp mill, all sulfate which becomes available in the production of chlorine dioxide. On the other hand, more and more stringent requirements are being imposed as regards the quality of pulp and this necessitates a higher consumption of chlorine dioxide, which is essential for a production of high-strength fibers and for a high brightness. For these reasons, conventional processes of producing chlorine dioxide result in increasing quantities of surplus sodium sulfate.
In the so-called Munich process of producing chlorine dioxide which is known from German Patent No. 831,542, chlorate is reacted with acid in a quantity which is not in excess of that which is stoichiometrically required, and the residual solution, which contains sodium chlorate, is recycled to the electrolytic process for producing chlorate. In said Munich process, a continuous stream of a sodium chlorate solution which contains sodium chloride flows at elevated temperature through a plurality of successive reaction vessels, preferably through six of such vessels, and is reacted therein with hydrochloric acid. The resulting mixed gases consisting of chlorine dioxide and chlorine are diluted with an air stream and are sucked off to ensure that the chlorine dioxide content in the reactor gas remains below the exposition limit of 15 percent. When the reaction solution has been concentrated by evaporation, it contains sodium chlorate and a large quantity of sodium chloride and is then fed to the electrolytic process for producing chlorate. The resulting chlorate solution is returned to the cycle. When chlorine dioxide from the mixture of chlorine dioxide, chlorine and air leaving the chlorine dioxide reactor has been absorbed, hydrochloric acid can be produced in that the chlorine contained in said mixture can be burnt with the hydrogen which has been produced in the chlorate-producing electrolytic PG,4 process. Obviously, the quantity of hydrochloric acid which is thus produced is less than is required for the production of all chlorine dioxide.
The process which has been described involves another difficulty, in that the dilution with air must be very closely controlled in order to enable a combustion of chlorine with atmospheric oxygen. On the other hand, a supply of air at a lower rate would involve a rather close approach to the explosion limit of chlorine dioxide.
A further disadvantage of that process resides in that hydrochloric acid is not available or is in short supply in non-industrialized countries and a transportation of hydrochloric acid is expensive because hydrochloric acid is commercially available usually only as a 32 percent aqueous solution.
It is also known to produce chlorine, sodium hydroxide solution and hydrogen by an electrolysis of sodium chloride solutions in membrane cells, in which an anode chamber and a cathode chamber are separated by an ion exchange membrane, which is permeable virtually only to the alkali ions. The latter are electrically neutralized at the cathode and react in the cathode chamber with water to form alkaline solution and hydrogen. On the other hand, halogen ions cannot migrate through the membrane and are released only in the anode chamber as halogen gas. Part of the OH ions formed in the cathode chamber migrate through the membrane into the anode chamber and in the latter react with the chlorine gas to form oxyacids of chlorine and/or salts thereof, depending on the pH value of the anolyte. These compounds are undesirable and can be destroyed only by an addition of acid. This disadvantage has been eliminated by the use of acid anolyte (D. Bergner, "Elektrolytische Chlorerzeugung nach dem Membran-Verfahren", Chemiker-Zeitung 101 (1977), pages 433 to 447). In the process disclosed in U.S. Pat. No. 3,948,737, the pH value of the anolyte should not be in excess of 4.5, and should preferably be between 2.5 and 4.0, although pH values of 1 or lower are permissible.
It is an object of the invention to provide for the production of chlorine dioxide and, if desired, chlorine, a process in which the use of alkali chlorate and hydrogen chloride as starting materials is avoided and which can be carried out to advantage in pulp mills which lack a satisfactory supply of raw materials.